1. Field of the Invention
The present invention relates to a signal processing method and device for image signal processing, and more particularly, for generating a new component image signal from an inputted component image signal consisting of a luminance signal and color difference signals, which causes no loss in image quality (especially, quality of the image representing the natural picture) when the image data are reduced through a proper filtering processing (high-frequency signal emphasis processing like high pass filtering for a luminance signal and high-frequency signal cutoff processing like low pass filtering for color difference signals in order to adapt the image signals to the visual characteristics of the human eyes), data thinning in a unit of time and the like.
2. Description of the Background Art
An image signal is generally formed of a luminance signal Y and color difference signals U, V, W. For example, assuming now that three signals (R-Y), (B-Y) and (G-Y) are adopted as the color difference signals U, V and W, the relation between these color difference signals and the luminance signal Y is expressed as EQU Y=0.30R+0.59G+0.11B (1) EQU R-Y=0.70R-0.59G-0.11B(=U) EQU B-Y=-0.30R-0.59G+0.89B(=V) EQU G-Y=-0.30R+0.41G-0.11B(=W) (2) EQU 0.3(R-Y)+0.59(G-Y)+0.11(B-Y)=0 (3)
From three signals, e.g., the luminance signal Y and the two color difference signals R-Y(=U) and B-Y(=V), reproduction of a full-color image signal consisting of red (R), green (G) and blue (B) can be achieved by using the above Formulae (1), (2) and (3). The most common application is an NTSC (National Television System Committee) color television signal. In short, the full-color information can be transmitted with a signal combination of the luminance signal Y and any two out of the color difference signals (R-Y), (B-Y) and (G-Y).
Now, additional discussion of the component image signal and a component coding system associated therewith will be given. The component coding is a system of independently converting RGB signals, the color difference signals (R-Y, B-Y) or the like which constitute the image signal, into digital codes. In the component coding, an input image signal is called a component image signal. Since the component coding would solve the problem of the difference in modulation system, discussion of a coding system has been held in European countries where there are a variety of modulation systems.
Under these circumstances, a challenge to be solved is how to reduce the transmission rate (the number of bits to be transmitted per unit of time, normally expressed in a unit of Mb/S; Mega bits per Second) or the number of data elements per specified pixel (i.e., the amount of data) in data transmission or memory processing of the above-discussed component signals U, V by using a component coding system. Assuming that the luminance signal Y and the color difference signals U, V are digital signals, three kinds of component coding systems have been considered possible, in which the transmission rates of the digital signals are as follows. Brief discussion thereof will be given below (taking the coding systems which stand in clear contrast to the present invention by a way of example). EQU (A) Y:U:V=4:4:4(=1:1:1) (4) EQU (B) Y:U:V=4:2:2 (5) EQU (C) Y:U:V=4:1:1 (6)
The coding system (A), in which all of the transmission rates (or the number of data per specified pixel) of the digital data of the luminance signal Y and the color difference signals U, V are equal, is a basic type of coding system. In the coding systems (B) and (C), the transmission rate (or the number of data per specified pixel) of the color difference signals is one half and one quarter the transmission rate (or the number of data per specified pixel) of the luminance signal Y, respectively, in data transmission or memory processing.
Obviously, with respect to the image quality represented by the color difference signals, the coding system (A) is the best, the coding system (B) is worse, and the coding system (C) is the worst. However, because of the visual characteristics of the human eyes that the resolution power to color is lower than the resolution power to the luminance signal, the coding systems (B) and (C) may be also used with no practical problem.
FIG. 7A shows an example of the coding system of 4:2:2 (cf. The Digital Circuit of Television Signal, by Etoh and Achiha of Corona Corporation, Sep. 25, 1989, the 1st edition, pp 8-10).
According to the parameters of FIG. 7A, the transmission rate (Mb/s) in serial transmission is 216 Mb/s (=8 bits.times.13.5M+8 bits.times.6.75M.times.2) and the transmission rate in parallel transmission with 9 transmission lines consisting of 8 data lines and 1 clock line is 27 MH (=13.5M+6.75M.times.2) per line. Practically, a redundant bit is needed. Assuming now that 1-bit redundant bit is needed with respect to the 8-bit data, the transmission rate is 243 Mb/s (=9.times.13.5M+9.times.6.75M.times.2) in serial transmission.
When two transmission lines for the luminance signal and the color difference signal (where the redundant bit is omitted) are separately provided in parallel, it is found from the above calculation that the transmission rates of the luminance signal and the color difference signals (U, V) are each 108 Mb/s. In addition, a clock transmission line of 108 Mb/s is also needed. Anyway, transmission lines to assure the transmission rate of around 100 Mb/s are required (in parallel transmission). This coding system (4:2:2) is the standard of recommendation of the CCIR (Consultive Committee International Radio) plenary session, which is used for the D1-format of the digital television studio and of the digital VTR.
Furthermore, for reference, FIG. 7B shows an HDTV (high definition television) coding system (cf. the above-referenced document, p10), in which the required transmission rate in parallel transmission is 148.5 Mb/s (=74.25M+37.125M.times.2) and the required transmission rate in serial transmission is as much as 1188 Mb/s (=148.5M.times.8).
As to the coding system (C) of 4:1:1, when two transmission lines are separately provided in parallel as above (where the redundant bit is omitted), the transmission rate of the luminance signal is 108 Mb/s and the transmission rate of the color difference signals (U, V) is 54 Mb/s.
When the transmission rate (or the number of data, the amount of data per specified pixel) of the color difference signals is lower (e.g., 4:0.5:0.5=8:1:1) as compared with the coding system (C) of 4:1:1, obviously the image quality is worse.
In this case, however, there arises advantages such as reduction in the amount of data on the whole, more specifically, reduction in the amount of used memory, downsizing in circuit scale and reduction in power consumption which are expected in a digital video, and further, reduction in transmission time for a prescribed amount of information (e.g., a frame of data) with reduction in the amount of data when the same transmission rate is used. (Or, more amount of redundant data, character data and decoded data can be transmitted in a prescribed time.)
In the background art, considering the above advantages with no loss in image quality, the coding system of 4:1:1 has been used as a practical one.
FIGS. 6A, 6B and 6C illustrate models of pixel structure in the coding systems of 4:4:4, 4:2:2 and 4:1:1, respectively.
A typical model of the signal processing method and device in the background art merely inputs a component image signal consisting of an input luminance signal Y and input color difference signals (R-Y, B-Y) and outputs an output luminance signal Y and output color difference signals (R-Y, B-Y), and performs no particular signal processing for the luminance signal Y and color difference signals U, V themselves besides a prescribed filtering processing associated with the sampling frequency ratio of 4:1:1. As a matter of course, it is well known that conversion of the coding system of 4:4:4 into the coding system of 4:2:2 or of 4:1:1 is performed in a signal processing block as shown in FIG. 6D and the like.
As discussed above, considering reduction in the transmission rate or the number of data per specified pixel (the amount of data) with no loss in image quality, the coding system in which the ratio of the transmission rates (or the amount of the image data) of the luminance signal Y and color difference signals (U, V) is 4:1:1 has been generally used in the background art. There has been a problem that the attempt to further reduce the color difference signal data results in loss in image quality of finally reproduced full-colored image.